Variable cylinder hydraulic vibrator and control system

ABSTRACT

Apparatus for incremental cylinder length control in hydraulic seismic vibrators having increased frequency range. The vibrator apparatus (10) of the type having a reaction mass (38) and axial cylinder bore (42) utilizes hydraulically movable end sleeves (86 and 96) in each end of the cylinder bore (42) so that the end sleeves (86 and 96) can be controllably positioned thereby to alter the cylinder volume and adjust the hydraulic drive system (18) for maximum efficiency relative to operating frequency.

DESCRIPTION BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to seimsic energy vibrators and, moreparticularly, but not by way of limitation, it relates to an improvedvibration apparatus having selectively controllable drive cylindervolume for frequency range adaptability.

2. Description of the Prior Art

Prior art in point is U.S. Pat. No. 4,106,586 in the name of Stafford,property of the present assignee, which teaches a hydraulic siesmicvibrator wherein the reaction mass includes an axial drive cylinder thatis variable between two positions, a maximum and a minimum cylindervolume. On the order of the present invention, the Stafford vibratorutilizes movable end sleeves disposed in opposite ends of the hydrauliccylinder and these, in turn, are hydraulically controllable between twopositions, simultaneously, relative to the height of the cylinder. Thepresent invention is directed to a continuously variable cylinder volumecontrol and the capability of achieving optimum fluid compressibilityfactor throughout a selected frequency range of operation, and this isparticularly desirable at higher seismic frequencies on the order of 250Hertz and up.

SUMMARY OF THE INVENTION

The present invention contemplates a hydraulic seismic vibrator having areaction mass driven on a double rod-end piston that is frame supportedto couple selected frequency vibrational energy through a suitablebaseplate for propagation of seismic energy within an earth medium. Thedouble-rod-end piston is reciprocally disposed within an internal drivecylinder within the reaction mass, and hydraulic servo control providescontrolled frequency alternating fluid under pressure to drive thereaction mass. Slidable end sleeves are disposed in opposite ends of theinternal cylinder and hydraulically controllable either manually or inresponse to sweep control input for variation of their relativepositions thereby to change the internal maximum length of the drivecylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an idealized view in elevation, with parts in section, of ahydraulic vibrator constructed in accordance with the present invention,a control system being shown in block diagram; and

FIG. 2 is an idealized view in elevation, with parts shown in section ofan alternative form of seismic vibrator and cylinder volume control.

DETAILED DESCRIPTION

FIG. 1 illustrates a compressional wave seismic vibrator 10 placed inenergy coupling engagement by a baseplate 12 for propagation of energyinto an earth medium 14. The vibrator 10 is controlled as to range andsweep frequency by a hydraulic drive control system 18. While notspecifically shown, the basic sweep control signal generator for thevibrator system is well-known in the art, and in this case it providesinput of sweep control signal 20 for the cylinder length control as wellas the control signal input to hydraulic drive control system 16.

Referring to vibrator 10, a circular top plate 22 is rigidly secured ata center recess 24 to an upper end 26 of a double-rod-end piston 28. Innormal practice, top plate 22 is secured to piston rod 26 by concentricbolting. In like manner, a circular lower plate 30 is bolt-secured atcentral recess 32 to receive a lower end 34 of double-rod-end piston 28.A cylindrical side wall 36 is then secured as by welding to enclose areaction mass 38, the entire housing structure providing a rigidconnection between upper and lower rod ends 26 and 34, respectively, toa selected baseplate 12 that provides energy coupling function incontact with earth surface 14.

The central axis of reaction mass 38 is then formed with a central bore40 through which the double-rod-end piston 28 is received in sealed,reciprocable relationship. The bore 40 includes a larger diametercentral portion forming a drive cylinder 42, an enlarged diameter uppercylinder 44, and another enlarged diameter lower cylinder 46. Thus,upper and lower cylinders 44 and 46 are characterized by upward anddownward facing shoulders 48 and 50, respectively, as they are disposedon opposite ends of drive cylinder 42. The upper end of bore 40 issealingly closed by an end cap 52 secured to reaction mass 38 by meansof a plurality of concentric bolts, 54, and having a central opening 56carrying annular seal 58 in contact around upper-rod-end 26. In likemanner, the lower end of reaction mass 38 has an end cap 60 as securedby bolts 62 to maintain annular seal 64.

Thus, the double-rod-end piston 28 is sealingly received within reactionmass 38 and includes a central piston member 66 carrying a plurality ofring seals 68 in contact with the surface of drive cylinder 42.Hydraulic drive pressure is applied to upper portion 70 within cylinder42 through the fluid passage 72 and port 74 as supplied from a top platefeedthrough connector 76 further connected to the servo valve (notspecifically shown) within hydraulic control system 16. In usualconstruction, the servo valve is rigidly secured on or about vibrator10. Fluid pressure to the lower portion 78 of drive cylinder 42 issupplied via passage 80 and port 82 from a similar feedthrough connector84 and the servo valve.

The length of upper portion 70 of drive cylinder 42 is controlled by amovable end sleeve 86, a body of revolution having a central bore 88, alarge diameter portion 90 and lesser diameter portion 92 is separated bya shoulder 94. Similarly, a lower end sleeve 96 has a central bore 98,for sealingly receiving lower rod-end 34, as well as a large diameterend 100 and lesser diameter end 102 as separated by seating shoulder104.

Upper end sleeve 86 is adapted to be hydraulically driven between limitsof end cylinder 44 and includes sealing rings 106 and 108 belowrespective fluid porting passages 110 and 112 so that the end sleeve 86may be selectively or automatically positioned within its lower surface114 defining the upper limit of volume portion 70 of drive cylinder 42.In like manner, and as controlled in synchronism, lower end sleeve 96carries slidable seals 116 and 118 above respective fluid passages 120and 122 as end sleeve 96 is controllably slidable to position topsurface 124 at a selected lower limit for drive cylinder 42.

Fluid passage ways 110 and 112 are connected to a servo valve 126 which,in turn, is connected through flexible hose loops 128 and 130 andrespective feedthrough connectors 132 and 134 to the hydraulic pressuresupply lines 136 and 138 from pressurized hydraulic fluid source 140.Similarly, lower fluid control passages 120 and 122 are connectedthrough a servo valve 142, flexible hose loops 144 and 146, andfeedthrough connectors 148 and 150 to the fluid control lines 136 and138. The servo valves 126 and 142 may be any of various commerciallyavailable types, e.g. Moog Type AO 76-104.

Position sensing feedback is carried out by linear velocity differentialtransformers, known as LVDT sensors. The position of both end sleeves 86and 96 are continually sensed to provide a position signal output toassociated comparators in control system 18, as will be furtherdescribed. Thus, an LVDT 152 is suitably mounted through upper end plate52 and a portion of reaction mass 38 to extend a sensing core 154 downwithin a bore 156 formed in upper end sleeve 86. Signal output from LVDT152 is provided via lead 158 to provide position signal. In like manner,the position of lower end sleeve 96 is sensed by an LVDT 160 extending asensing core 162 into a similarly formed bore 164. Output positionsignals are then provided by electrical lead 166. A suitable form ofcommercially available LVDT is the SCHAEVITZ LVDT, MODEL 1000.

The control system 18 may function either on manual or automatic controldepending upon the setting of switch 168. Thus, in the manual position,position selector 170 may be set to provide manual control voltagethrough 168 to a lead 174 which supplies the reference voltage inparallel to each of the two comparators 174 and 176. The comparators maybe described as position servo systems where a position feedback signalis compared against a reference position signal and provides an error orsignal drive to the servo valve until the feedback signal is equal tothe reference signal. A feedback position signal from LVDT 152 issupplied on lead 158 to the other input of comparator 174, while asimilar feedback position signal from LVDT 160 is provided on lead 166to the second input of comparator 176. Correction or error voltageoutput from each of the comparators 174 and 176 will then be present onrespective leads 178 and 180, through suitable feedthroughs of side wall36 to the respective cylinder length servo valves 126 and 142.

When in the automatic mode setting of switch 168, sweep control signal20 is provided as input to a frequency to voltage converter 18 whichthen derives a reference voltage for conduction via switch 168 and lead172 for input to the comparators 174 and 176. The comparators 174 and176 will then derive a comparison voltage relative to the sweep controlreference signal on lead 172 to provide error voltage output onrespective leads 178 and 180 thereby to control cylinder length servovalve 126 and 142. The control system 18 may be constructed either as ana-c or a d-c system; that is, a d-c system may be provided whereinposition selector 170 and/or converter 167 provide a d-c outputreference voltage and, accordingly, the LVDT's 152 and 160 would be d-ctype units providing d-c error voltage on the respective leads 158 and166. All such circuitry is entirely conventional and a matter of choiceof components. One may utilize a-c electronics to carry out the samecontrol functions.

FIG. 2 illustrates an alternative form of construction as a shear waveseismic vibrator 190 is operative through a baseplate 194 to coupleseismic energy vibrations into earth medium 14. The vibrator 190 mayutilize the same essential control system 18 to effect electromechanicalcontrol of the drive cylinder volume for shear wave vibrator 190. In thesame manner as the FIG. 1 embodiment, vibrator excitation is effected byapplication of sweep control signal 20 through vibrator controlcircuitry 196 to provide control signal output on lead 198 to a servovalve 200. Servo valve 200 then tracks to selectively apply hydraulicfluid pressure from hydraulic source 140 through respective portingpassages 202 and 204 formed within reaction mass 206 and incommunication with opposite sides of a drive cylinder 208.

Vibrator 190 includes the reaction mass 206 as reciprocally disposed ona double-rod-end piston 210 having opposite ends 212 and 214 as arerigidly secured within end frames 216 and 218. In turn, the end frames216 and 218 are rigidly secured to a base member 220 which is furthersecured to baseplate 194. Reaction mass 206 includes a central, axialbore similar to that of reaction mass 38 of FIG. 1, and controllablyslidable end sleeves 222 and 224 are sealingly disposed for synchronizedmovements to control the internal length of drive cylinder 208. However,in this case the movement of end sleeves 222 and 224 is effected bymeans of a screw jack mechanism. Thus, end sleeve 222 may be reversablydriven by a drive screw 226 within a threaded bore 228 receivingrotation from a shaft 230 and electrical motor 232 .[.mounted on endframe 216.]. .Iadd.Motor 232 should not be mounted on end frame 216. Anappropriate mounting of the motor would be apparent to one skilled inthe art. .Iaddend.The opposite end sleeve 224 may be similarly moved bya drive screw 234 within threaded bore 236 as it receives rotation viashaft 238 from motor 240. In this case, comparators 174a and 176a wouldinclude power amplifier circuitry assuring adequate control power oncontrol leads 178a and 180a to the respective drive motors 232 and 240.

In operation, and referring primarily to FIG. 1, the end sleeves 86 and96 are each formed with a lesser diameter end that can be controllablymoved into or out of drive cylinder 42 thereby to increase or decreasethe cylinder length therein. The opposite ends of end sleeves 86 and 96have a larger diameter, thus greater area, and as hydraulic fluid isported through the large area end of control cylinders 44 and 46,respective end sleeves 86 and 96 will move into the drive cylinder 42 todecrease the cylinder length. This has the effect of increasing thefluid compressibility factor which is a desirable end in controlling theseismic vibrator 10 toward the higher frequencies, i.e. 250 Hertz andhigher. Alternatively, and in accordance either with position selector170 or sweep control signal 20, fluid porting to the lesser area end ofthe end sleeves 86 and 96 will cause the respective end sleeves to moveout of the drive cylinder 42 allowing greater cylinder length as isdesirable for the lower frequency vibrations. In this case, there is adecreasing compressibility factor and a lower fluid mass breakfrequency.

The position of the end sleeves 86 and 96 is continually sensed by theposition sensors or LVDT 152 and 160, and the respective sensed positionsignals on leads 158 and 166 are fed to the respective comparators 174and 176 along with a required position reference signal that is appliedthrough switch 168, i.e. from either the manual position selector 170 orthe automatic tracking signal as derived from sweep control signalgenerator 20 through frequency to voltage converter 167. The outputerror signal from each of comparators 174 and 176 is then fed via leads178 and 180 to the respective control servo valves 126 and 142 whichthen port hydraulic fluid to the end sleeves 86 and 96 thereby toestablish the required end sleeve position. Thus, it can be seen thatthe cylinder interval length can be set by manual selector 170 for agiven frequency of operation, or the cylinder length can be controlledautomatically to maintain a preselected high compressibility factor atall frequencies within the range of vibrator control signal frequenciesas derived from sweep control signal generator 20. In the automaticmode, maximum energy coupling and efficiency is enabled as the endsleeve positioning is made to track with the vibrator control frequencysweep.

In FIG. 2, the same essential operation is effected albeit in the caseof a shear wave vibrator 190, as selected for illustration. It isforeseen that it will be more difficult to operate the mechanicalcontrol system of vibrator 190 in an automatic mode or on a fast sweepfrequency; however, it may be quite desirable for some field operationswherein many shots are made with a given frequency progression or rangeof frequencies, and periodic manual positioning of the electromechanicalsystem will enable achieving of maximum energy coupling into the earthat the selected frequencies.

The foregoing discloses a novel form of variable cylinder lengthhydraulic vibrator and it is believed that changes may be made in thecombination and arrangement of elements as heretofore set forth in thespecification and shown in the drawings; it being understood thatchanges may be made in the embodiments disclosed without departing fromthe spirit and scope of the invention.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Apparatus forincremental cylinder length control in a hydraulic vibrator of the typehaving a vibrating reaction mass with a cylinder bore disposed therein,comprising:first end sleeve means disposed in sealed, movable positionin one end of said cylinder bore; second end sleeve means disposed insealed, movable position in the other end of said cylinder bore anddefining a cylinder of predetermined length between said end sleevemeans; piston rod means sealed and slidably disposed through said firstand second end sleeve means and including a piston disposed in saidcylinder bore; hydraulic means for selectively positioning said firstand second end sleeve means to adjust said cylinder bore lengthincrementally; sensor means disposed adjacent said end sleeve means togenerate a sleeve position signal; control selection means generating areference signal; means comparing said position and reference signal forgenerating an error signal for controlling said hydraulic means; andmeans for hydraulically actuating said piston for reciprocation withinsaid cylinder thereby to impart vibration to said reaction mass. 2.Apparatus as set forth in claim 1 wherein said hydraulic means forselectively positioning comprises:first and second hydraulic servo valvemeans each communicating through said reaction mass to positionrespective ones of said first and second end sleeve means relative tosaid piston.
 3. Apparatus as set forth in claim 2 wherein said sensormeans, control selection means and means comparing comprise:first sensormeans disposed proximate said first end sleeve means to generate a firstsleeve position signal; second sensor means disposed proximate saidsecond end sleeve means to generate a second sleeve position signal;control selection means generating a reference signal; and first andsecond comparator means each comparing respective first and secondposition signals to said reference signal to generate first and seconderror signals for connection to control said first and second hydraulicservo valve means.
 4. Apparatus as set forth in claim 3 wherein saidcontrol selection means includes: means for enabling a selected voltagereference as a continuous selection signal.
 5. Apparatus as set forth inclaim 3 wherein said control selection means includes: means responsiveto vibration frequency of said reaction mass for generating a selectionsignal wherein the voltage reference varies in proporation to saidvibration frequency.
 6. .[.Apparatus as set forth in claim 1.]..Iadd.Apparatus for incremental cylinder length control in a hydraulicvibrator of the type having a vibrating reaction mass with a cylinderbore disposed therein, comprising:first end sleeve means disposed insealed, movable position in one end of said cylinder bore; second endsleeve means diposed in sealed, movable position in the other end ofsaid cylinder bore and defining a cylinder of predetermined lengthbetween said end sleeve means; rod means sealed and slidably disposedthrough said first and second end sleeve means and including a pistondisposed in said cylinder bore; means for selectively positioning saidfirst and second end sleeve means to adjust said cylinder bore lengthincrementally; sensor means disposed adjacent said end sleeve means togenerate a sleeve position signal; control selection means generating areference signal; means comparing said position and reference signal forgenerating an error signal for controlling said positioning means; meansfor hydraulically actuating said piston for reciprocation within saidcylinder thereby to impart vibration to said reaction mass; .Iaddend.wherein said means for selectively positioning comprises: first andsecond screw means disposed for threaded rotation in each of said firstand second end sleeve means, first and second reversible motor meansdisposed proximate said reaction mass to provide rotational drive tosaid respective first and second screw means; and control meansconnected to provide energizing control to each of said first and secondreversible motor means.